The present invention relates to a power supply arrangement, and particularly to such an arrangement for generating a modulated voltage for the anode of the final stage of a power transmitter.
A high frequency power transmitter which has an output power of, for example, 500 kW and a high frequency final stage tube with a modulated anode voltage must be supplied with an anode voltage of approximately 30 kV that has been amplitude modulated with a low frequency (LF) signal. It is desirable that such a power transmitter has the best possible efficiency.
Switching amplifiers are known which sum a plurality of switchable direct voltage signals along a diode cascade so that a desired high output voltage is produced. If the individual direct voltage signals are switched in dependence on the magnitude of an LF signal, the high voltage output signal approximates the LF signal amplified by a desired amount. Such a switching amplifier includes a power transformer which has at least one primary winding and a plurality of secondary windings that are not electrically connected with one another. Such a configuration produces so-called floating direct voltage sources on the secondary side which generate essentially identical output voltages which are then summed as discussed above.
Such a summation is illustrated diagramatically in FIG. 1 which shows voltage U as a function of time t. The curve marked U.sub.A represents the amplitude modulated anode voltage to be produced. This voltage is approximately produced by the exemplary summation of the individual direct voltages U.sub.1 to U.sub.5 which are essentially equal in magnitude.
The voltage summation according to FIG. 1 is disadvantageous because it results in very unequal load times for the individual direct voltage sources. For example, the direct voltage source belonging to the individual direct voltage U.sub.1 must be switched on at time t.sub.1 and switched off at time t.sub.10, while the direct voltage source belonging to individual direct voltage U.sub.5 is not switched on until time t.sub.5, which is significantly later and is switched off at time t.sub.6 which is significantly earlier. In this example, the direct voltage source belonging to individual direct voltage U.sub.1 is under load approximately twice as long as the direct voltage source belonging to individual direct voltage U.sub.5.
It is of course possible to construct the individual direct voltage sources in such a manner that they are adapted to the differences in load. However, such an adaptation is uneconomical in industrial mass production and thus clearly a disadvantage.
To avoid this drawback, the direct voltage sources can all be designed to accommodate the greatest possible load. Such a structure, however, is also uneconomical to produce.
It is further desirable to keep the magnitude of the individual direct voltages as low as possible in order to minimize the step structure shown in FIG. 1 and thus to realize a modulated anode voltage which has relatively low distortion.
European Patent Application No. 0,124,765 discloses a monitoring device which monitors the operating parameter values of individual, series-connected power supply units and feeds these values, with the aid of a feedback loop, to a computer or actuation unit which then makes the decision as to which power supply unit is to be switched on, and when and for how long. The purpose of this device is to keep the loads on all power supply units as uniform as possible so as to minimize the loads on these units.
The drawback here is that the calculations performed for the selection of the individual power supply units considers only the optimum loads and not the electrical geometry of the transformer employed. Moreover, this implementation also involves considerable expenditures to measure the operating parameters for each power supply unit and to make a decision as to which individual power supply unit is to be switched.